Pore structure of gel chitosan membranes. I. Solute diffusion measurements

The capillary pore model of water-swollen gels was used to interpret solute diffusion through gel chitosan membranes. Diffusive permeability coefficients of 12 solutes ranging in molecular radius from 2.5 Å (methanol) to 14 Å (polyethylene glycol 4000) were measured for an untreated chitosan membrane, for four chitosan membranes crosslinked with glutaraldehyde of concentrations between 0.01 and 1% and coated with a protein and also, for comparison, for a commercial Cuprophan membrane. Through the capillary pore-model correlation of the above coefficients with the membrane water content, the following structural factors of the examined membranes were calculated: pore radius, surface porosity and tortuosity factor. Knowledge of these factors is required if the desired membranes are to be designed for a given application (e.g. dialysis).     

Ion transport across membranes prepared by gel crystallization

A one-step procedure for the preparation of ion-selective membranes is described. The method employs the thermally induced gel crystallization of ultrahigh molecular weight poly(ethylene) (UHMW-PE) from a dilute xylene solution. After evaporation of the xylene, a microporous UHMW-PE film remains, which can serve as the support for liquid and polymeric ion-selective membranes. The addition of a membrane solvent and suitable receptor molecules to the xylene solution allows a one-step incorporation of these membrane components into the UHMW-PE support. The influence of the preparation conditions of the UHMW-PE support on the rates of the p-tert-butylcalix[4]arene tetraethylester-mediated transport of NaClO₄ was studied. Two concepts to improve the life-time of the membranes are introduced. In a first approach, the addition of photocrosslinkable nitril-substituted siloxane copolymers to the membrane phase has been evaluated. The enhanced viscosity of the membrane phase reduces leaching of carrier and solvent molecules from the membrane into the aqueous phases. In a second approach, the solvent is omitted and the membrane-phase merely consists of a benzo-15-crown-5 or calix[4]arene modified siloxane-copolymer, which is substituted to such a degree that ion transport no longer has to take place via diffusion of host–guest complexes but by jumping of the cations from one fixed carrier to a neighboring carrier. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 383–394, 1998     

Computer program for simulation of mass transport in nanofiltration membranes

A computer program, NanoFiltran, was developed to simulate the mass transport of multi-ionic aqueous solutions in charged nanofiltration (NF) membranes, based on the Donnan steric partitioning pore and dielectric exclusion (DSPM&DE) model, with incorporation of the non-ideality of electrolyte solutions and concentration polarization effects in the membrane/feed-solution interface. With this computer program, the extended Nernst–Planck (ENP) equations are discretized inside the membrane, using the finite-difference scheme. The discretized ENP equations together with the other model equations are linearized in order to obtain a system of equations that are solved simultaneously. The linearized system of equations is based on an initial guess for the electrical potential and ions concentrations profiles, which are updated iteratively. A robust method of under-relaxation of the electrical potential and ions concentrations ensures that the convergence is achieved even for NF systems that exhibit a very stiff numerical behaviour.     

3D topography design of membranes for enhanced mass transport

This work describes the process of fabrication of 3D topography membranes and the fully quantitative characterisation of their topography using atomic force microscopy (AFM), small-angle light scattering (SALS), scanning electron microscopy (SEM) and polarizing optical microscopy (POM). The use of these membranes and the impact of the 3D membrane topography on the enhancement of mass transport during solute recovery (hexyl acetate) from a viscous room temperature ionic liquid 1-n-butyl-3-methyl-imidazolium tetrafluoroborate ([C₄mim] [BF₄]) by organophilic pervaporation is presented and discussed.     

Characterization of the pore area distribution in porous membranes using transport measurements

An approach originally proposed by Mason and coworkers has been applied to model porous membranes to show that transport measurements with small and large solutes can be used to distinguish between porous membranes with the same average pore size but different pore size distributions. In addtion, it is shown that such measurements can be used to account for membrane heteroporosity when predicting the sieving characteristics of a membrane. This is done by applying moment theory to results from flux measurements for a small solute at Pe ≈ 1 or to results from measurements of the reflection coefficient for a large solute at infinite Pe. No a priori assumptions about the nature of the distribution of pore areas are necessary.In this paper, the results from calculations performed with three different model membranes with log-normal pore size distribution are reported. These results show that one can begin to distinguish between membranes by measuring the hydraulic and diffusive permeability and performing at least one additional flux measurement — with either a small, non-hindered solute at Pe ≈ 1 or a large solute at infinite Pe. Results also show that a fairly narrow window can be placed on the sieving curve for a heteroporous membrane without performing any sieving measurements. This is an interesting and encouraging result because it means that many of the problems that arise from measuring and interpreting pore size distributions using more traditional techniques can be avoided by using small solute flux measurements to predict the separation characteristics of many porous membranes.     

A new apparatus and procedure for gas permeability measurements on water-swollen gel membranes

An improved apparatus for the measurement of gas permeability through water-swollen gel membranes is described. The apparatus is fitted with a water bubble chamber and a water reservoir to prevent evaporation of water from the swollen membranes. Permeabilities to O₂ and N₂ of two kinds of poly (vinyl alcohol) (PVA) membranes and cationically modified PVA membranes with a swelling ratio of 0.3–0.8 were measured. It was found that the introduction of charged groups at 3.5 mol/100 mol monomer units into the PVA membranes with the above swelling ratio range does not significantly alter the permeability of the membranes to the gases. The permeability values are independent of the upstream pressure up to 4.6 atm.     

Coupled transport membranes III: the rate-limiting step in uranium transport with a tertiary amine

This is the second paper describing the coupled transport of uranium ions through liquid membranes. The membranes consist of a microporous polymeric support with an organic solution of a tertiary amine complexing agent held within the pores by capillary forces. p]Both the composition of the organic solution and the structure of the microporous support have a marked effect on uranium flux. With increasing concentration of the agent in an inert diluent, both the amount of uranium that can be extracted into the membrane and the viscosity of the organic solution increase. These opposing effects result in a maximum flux at about 30 vol.% agent in the diluent. The size of the pores in the support also affects the flux; it appears that interaction with the pore walls in membranes with small pores hinders diffusion. p]Two types of interfacial effects have also been observed. The first of these is concentration polarization in the aqueous solution adjacent to the membrane surface. This effect can be reduced by increased stirring. Second, the transmembrane flux of uranium can be limited by the rate of formation (or dissociation) of the uranium complex at the membrane—solution interfaces.     

Preparation and drug retention of biodegradable chitosan gel beads.

Chitosan (CS) gel beads containing drug could be prepared in amino acid solutions of pH about 9, despite the requirement for a pH above 12 for gelation in water. This phenomenon was observed not only in amino acid solutions but also in solutions of compounds having amino groups. A solute concentration of more than 10% was required for preparation of gel beads at pH 9. Gelation of the CS beads required about 25 to 40 min, depending on the species of amino acid. Lidocaine hydrochloride (LC) as a model drug was retained in the beads to about 20 to 35% of the theoretical total amount, despite being a water-soluble drug. The release of LC from the CS gel beads was prolonged. The release pattern was not affected by the species of amino acid or CS, or the preparation time.     

Synthetic thermally reversible gel systems. III

Polyacrylylglycinamide (PAG) and its thermally reversible aqueous gels have been investigated and comparisons made with gelatin. For unfractionated PAG homopolymers in 2M NaCNS at 25°C, the Mark-Houwink-Sakurada equation is [η] = 1.16 × 10^?3 M?_n^0.52. The Huggins k′ value is found to be about 0.9 and the Flory-Huggins polymer-solvent interaction parameter, 0.49. A theoretically calculated value of K_θ is very close to the experimental one and it is for this reason and the observed μ and M?_n exponent values that 2M NaCNS at 25°C approaches being a θ solvent for PAG. A thermodynamic approach based on equilibrium swelling and modulus measurements indicates that a single thermally reversible crosslink in a gelatin gel involves numerous peptide backbone units, whereas in PAG gels a crosslink perhaps involves only one residue from each of two chains. These results complement the very high exothermic heat of gelation crosslinking, δH_c, for gelatin compared to PAG. ΔH_c has been measured on PAG samples of different DP and found to be independent of chain length. Similar measurements on acrylylglycinamide copolymers and terpolymers containing basic and acidic groups produce no change in δH_c so that coulombic forces do not appear to be significant. Data are also included relating to the density of PAG, its glass transition and thermal decomposition temperature, the tensile modulus of equilibrium-swelled PAG films in water and the chain-transfer constant of methanol with the acrylylglycinamide free radical.     

Temperature and concentration effects on electrolyte transport across porous thin-film composite nanofiltration membranes: Pore transport mechanisms and energetics of permeation

The influence of temperature and concentration on nanofilter charge density and electrolyte pore transport mechanisms is reported. Crossflow filtration experiments were performed to measure transport of several electrolytes (NaCl, NaNO3, NaClO4, CaCl2, MgCl2, and MgSO4) across two commercially available thin-film composite nanofiltration membranes in the range 5-41 degrees C. Experiments were also performed with selected salts in the range 1-50 meq/L to quantify concentration effects. Three different approaches, irreversible thermodynamics, extended Nernst-Planck formulation, and theory of rate processes, were employed to interpret retentions of these symmetric and asymmetric electrolytes at varying temperature and concentration. Increasing feed water temperature slightly increased electrolyte reflection coefficients and only weakly increased permeability compared with neutral solutes. Electromigration and convection tended to counteract each other at high fluxes explaining the weak temperature dependence of the reflection coefficient. Changes in membrane surface charge density with temperature were attributed to increased adsorption of electrolytes on the polymer constituting the active layer. Activation energy of permeation for charged solutes was primarily determined by the Donnan potential at the membrane-feed water interface. Electrolyte permeation was shown to be an enthalpy-driven process that resulted in small entropy changes. Increasing sorption capacity with temperature and low sorption energies indicated that co-ion sorption on polymeric membranes was an endothermic physicosorption process, which appears to determine temperature dependence of electrolyte permeation at increased feed concentrations.     

Modeling of mass transport with a very fast reaction through liquid membranes

In this paper a new model for mass transfer through liquid membranes is presented. The modeling is based on the assumption of a very fast chemical reaction, taking place inside droplets encapsulated within a liquid membrane and it permits the evaluation of mass transport kinetics. p]For computing the concentration in the continuous phase or the time required for the process, the following data are needed: diffusivity of the transported compound in the membrane phase, the ratio of the volume of dispersed to continuous phase, the diameter of the drops, the thickness of the membrane, and the equilibrium between the merebrane and the continuous phase. p]The model was checked in experimental measurements of phenol removal from water. Satisfactory agreement between experimental and theoretical results was found.     

Relationship between structure and transport in cellulose acetate desalination membranes

Several experimental techniques, including measurements of salt rejection, water flux, birefringence, and membrane opaqueness, were used to characterize the structure and performance of typical cellulose acetate desalination membranes. The results indicate that optimum salt rejection and water flux characteristics are related to the presence of molecular orientation effectively perpendicular to the membrane surface. An alignment-type flow mechanism involving water contained within the oriented structure is consistent with the observed structural and transport phenomena. The various membrane fabrication procedures apparently have significant effects on the development of molecular orientation in the thin surface layers of the membrane.     

Acetylation and molecular mass effects on barrier and mechanical properties of shortfin squid chitosan membranes

Chitosan samples with different N-deacetylation levels were obtained from β-chitin under heterogeneous alkali conditions. Oxidative depolymerisation was performed to attain low-acetylated chitosan samples with different molecular mass. Water vapour permeability, membrane swelling and tensile mechanical properties were analysed in plasticized self-supporting chitosan membranes. The main purpose was to describe unambigously the effect of the biopolymer molecular mass and acetylation degree on these properties. Commercially available chitosan samples derived from α-chitin were also studied for comparison. The equilibrium degree of swelling in water and the water vapour permeability increase by increasing the molecular mass or the degree of acetylation. Regarding the effect on the mechanical properties, generally harder and tougher membranes were obtained for chitosans with higher molecular mass or lower acetylation degree. These observations are tentatively explained based on the different structural characteristics of the polymer and can lead to a better understanding of the tools necessary to tailor a specific type of chitosan membrane.     

不同交联剂含量对戊二醛交联壳聚糖膜结构与性 能影响的研究

仔细研究壳聚糖膜的结晶度、溶胀度及其对二价铜离子的吸附量与交联剂戊二醛含量(特别是在戊二醛含量较低时)的关系。结果发现膜的结晶度、溶胀度以及对铜离子的吸附量均在戊二醛摩尔分数为0.25%时达到极大值。结晶度的增大可归结于轻度交联能使壳聚糖分子链在成膜时排列更为有序;而溶胀度和对铜离子吸附量的增加则可认为是交联能使壳聚糖中原先被氢键作用所束缚的氨基获得了自由。     

Carbon dioxide separation from hydrogen and nitrogen by fixed facilitated transport in swollen chitosan membranes

The separation of carbon dioxide from hydrogen and nitrogen at high temperatures would be valuable to fuel cell, flue gas purification, and ammonia processes. A feed gas mixture of carbon dioxide, hydrogen and nitrogen (10% CO₂, 10% H₂, and 80% N₂) was used to evaluate water-swollen chitosan as a facilitated transport membrane for these applications. The amino group of the chitosan repeating unit could be the fixed carrier that facilitates carbon dioxide transport in the presence of water.     

Analysis of chitosan by gel permeation chromatography

The effect of γ-radiation on the average molecular weight and the molecular-weight distribution of chitosan has been studied. The analysis of samples by gel permeation chromatography/size exclusion chromatography (GPC/SEC) has shown that the amount of high-molecular-weight fractions decreases and the amount of fractions containing low-molecular-weight chitosans (<50 kDa) increases with an increase in the radiation dose. The radiation-chemical yield of degradation is 11.0 particle/100 eV.     

Mg-ion conducting gel polymer electrolyte membranes containing biodegradable chitosan: Preparation,structural, electrical and electrochemical properties

A biodegradable gel polymer electrolyte system based on chitosan/magnesium trifluoromethanesulfonate/1-ethyl-3-methylimidazolium trifluoromethanesulfonate (CA/Mg (Tf)₂/EMITf) is developed. The structure, thermal performance, mechanical properties, ionic conductivity, relaxation time, electrochemical stability and ionic transport number of the membranes are analyzed by various techniques. The ion migration mainly depends on the complexation and decomplexation of Mg²⁺ with amine band (NH₂) in chitosan. The 90CA-10Mg (Tf)₂ system plasticized with 10% EMITf (relative to the amount of 90CA-10Mg (Tf)₂) is identified as the optimum one and the temperature dependence of ionic conductivity obeys the Arrhenius rule. Moreover, the relaxation time of the electrolyte is very short, being just 1.25 × 10⁻⁶ s, and its electrochemical stability window is quite wide, being up to 4.15 V. The anodic oxidation and cathodic reduction of Mg at the Mg-electrode/electrolyte interface is facile, and the ionic transference number of this electrolyte is 0.985, indicating that it could be a potential electrolyte candidate for Mg-ion devices.